1. Field of the Invention
The present invention relates to a process for producing a titanium-containing metal oxide the coloration of which is decreased. The present invention also relates to a hologram recording material suitable for volume hologram recording wherein a titanium-containing metal oxide the coloration of which is decreased is used as a metal oxide matrix, a process for producing the same, and a hologram recording medium having a hologram recording layer comprising the hologram recording material. The present invention relates particularly to a hologram recording material suitable for record and reproduction by use of not only a green laser light but also a blue laser light, a process for producing the same, and a hologram recording medium having a hologram recording layer comprising the hologram recording material.
2. Disclosure of the Related Art
The use of films derived from the sol-gel process has been spreading in order to modify the property of a polymer material surface or form an organic-inorganic hybrid film. When a metal oxide is formed by the sol-gel process, a hydrolyzable-group-containing compound of the corresponding metal, for example, an alkoxide compound of the metal is used as a starting material to conduct hydrolysis and polycondensation reaction in an appropriate solvent in the presence of an acid catalyst, thereby yielding a liquid sol, and then the polycondensation reaction is further advanced to turn the sol into a wet gel of the metal oxide. Thereafter, the gel is turned into a dry gel if desired. In the sol-gel process, a sol can be molded, or a sol is applied onto a substrate, thereby making it possible to form a gel film. Alternatively, a gel fiber can be formed by spinning, and various molded products can be produced by molding. In such a way, the sol-gel process advances at low temperature; thus, the process is an excellent process.
When an alkoxide compound of titanium is used as a starting material in the sol-gel process, an oxide of titanium, TiO2, can be obtained. TiO2 has a high refractive index so as to be optically advantageous. Moreover, TiO2 is used for surface modification since TiO2 has a photocatalytic effect.
However, TiO2 obtained by the sol-gel process undergoes coloration easily. Even if an alkoxide compound of titanium as a starting material is colorless, the compound is frequently colored into yellow at the stage when the compound reacts. TiO2 colored into yellow absorbs light in the blue region so as to cause a problem when the TiO2 is used in an optical waveguide or is used in a hologram recording material for a system wherein a blue laser is used to attain recording/reproducing. Moreover, a problem is caused when the TiO2 is used in any other article wherein the absorption of light in the blue region is not preferred. From this viewpoint, it has been desired to develop a producing process of a titanium-containing metal oxide the coloration of which is decreased.
Examples of the property required for a volume hologram recording material include high refractive index change at the time of recording, high sensitivity, low scattering, environment resistance, durability, low dimensional change, and high multiplicity. About holographic memory record using a green laser, various reports have been made hitherto as follows.
For example, JP-A-2005-321674 discloses a hologram recording material comprising: an organometallic compound at least containing at least two kinds of metals (Si and Ti), oxygen, and an aromatic group, and having an organometallic unit wherein two aromatic groups are directly bonded to one metal (Si); and a photopolymerizable compound. In Example 1 of the publication (in particular, pars. [0074] to [0078]), it is disclosed that a hologram recording medium which has a layer of the above-mentioned hologram recording material having a thickness of 100 μm gave a high transmittance, a high refractive index change, a low scattering, and a high multiplicity in record using a Nd:YAG laser (532 nm).
The above-mentioned publication discloses holographic memory record using a green laser, but does not disclose holographic memory record using a blue laser.